The invention relates to physically drying water-based coating agents, of use more particularly for single-colour or metallic-colour base layers of multi-layer lacquer coats.
Aqueous coating agents for producing base layers of multilayer coats are known. For example EP-A-0 089 497 describes water-dilutable coating agents based on polyurethane dispersions produced by a specified construction method from polyether diols and/or polyester diols, diisocyanates, compounds containing two groups which react with isocyanate and at least one anionic group, diamines and/or polyamines. These coating agents are stoved at an elevated temperature of e.g. 130.degree. to 150.degree. C., optionally after coating with a covering lacquer.
In industry and trade, however, there is a need for physically drying coating agents, suitable more particularly for repair lacquering of various substrates such as metal articles, particularly in the motor-vehicle sector, or for lacquering of machines. The coating agents required for these purposes must dry or harden at room temperature or with slight heating, e.g. to up to 60.degree. C.
Water-based physically drying coating agents of this kind, suitable e.g. for repair lacquering of motor vehicles, are known e.g. from DE-A-41 15 015 and DE-A-41 15 042. However, the coating agents described therein and based on polyurethane binders have disadvantages in multi-layer structures, compared with conventional solvent-containing base lacquers. More particularly the total structure has inadequate hardness.
DE-A-41 22 265 describes polyurethane dispersions which, after conversion into the aqueous phase, are suitable for producing metallic and single-colour base lacquers. The main disadvantages of these coating agents are their inadequate hardness in multi-layer structures and their water-tightness, which is inadequate under the conditions of vehicle repair lacquering.
DE-A-39 36 794 describes polyurethane polyureas and use thereof as binders in aqueous metallic base lacquers. The binders contain chemically incorporated carbonate groupings. Base lacquers on this basis, however, as compared with solvent-based base lacquers, have inadequate hardness in the total structure, particularly when covered with conventional clear lacquers.